Wireless local area networks (WLANs) comprise a network of wireless work stations which communicate with each other and a central file server via wireless communication. Because no wire connects the work stations to the network, a WLAN network is easier to install than conventional wired local area networks (LAN) and allows the work stations to be mobile. WLANs may be used, for example, in retail stores and factories, using palmtop computers as work stations to track inventory and the like. Another use is in hospitals, where wireless work stations may be taken to each patient's bed to keep track of medications given, vital signs, etc. One limitation of WLANs is that they are limited to a total network area of only several hundred yards. In order to increase the coverage of a WLAN, one or more WVLANs may be connected via an interface to a conventional wired LAN. This connection allows WLANs to cover larger areas, such as several floors in a building.
FIG. 1 illustrates a network 50 comprising a WLAN connected to a wired LAN. In this illustration, the wired LAN 52 is preferably an asynchronous transfer mode (ATM) network. ATM networks are preferred because they provide sufficient network bandwidth and easily integrate with existing networks, such as telephone and database networks. A person skilled in the art understands, however, that wired LANS using other protocols are also suitable. The LAN 52 has one or more switches 54, 56, 58 physically connected to it. Each switch 54, 56, 58 is physically connected to a base station (BS) 60a-60f. The base stations 60 are the interfaces between the LAN and WLAN. Each base station 60 defines an area of wireless communication coverage called a coverage area (represented in FIG. 1 as a circle) 62a-62f. A wireless work station, also called a mobile terminal (MT) 64, may be in wireless communication with the base station 60 covering the coverage area 62 in which the MT 64 is located. If the MT 64 travels from one coverage area to another, the network must be able to reroute communications to that MT 64 at its new location. This rerouting, known as a handoff, is preferably seamless, e.g., not apparent to the network users. The following three rules should be met for seamless handoffs:
(1) the data continuity should not be interrupted; PA1 (2) the handoff should be transparent to other users; and PA1 (3) the handoff should add only minimum loading to the network.
One method satisfying the first two rules is proposed in Goodman, et al., "Network Control for Wireless Communications," IEEE Comm. Mag. 116-24 Dec. 1992. This paper proposes a "path elongation" method.
This path elongation method provides for designating a virtual channel connection (VCC) between each pair of base stations in the WLANs and/or LANs (e.g., both wireless and wired networks). In known methods, a VCC is a virtual connection between two end stations (e.g., base stations). In an ATM system, a VCC is established before data is transmitted. An example of this method is as follows. A first MT wants to transmit a communication to a second MT. The second MT is moving from one coverage area to another, thus a handoff should be made. At the time of a handoff, the first MT transmits a wireless transmission to its current BS. This transmission is in a frame format used in wireless communications. The first MT's base station translates the communication from the frame format into a cell format used in ATM communications. The BS in which the second MT was just previously located receives the communication and translates it from a cell to a frame format. The previous BS is aware that the second recipient MT has moved to a new location. The BS translates the communication from the frame to the cell format and transfers the communication to the second MT's new BS along a newly established VCC between the base stations. The second MT's new BS then reassembles the cell format communication into a frame format communication and transmits it to the second MT.
When the second MT enters a new coverage area, it changes the database inside itself to reflect the new BS as the one in which it is currently located. The second MT also notifies the new BS of the identity of the previous BS. The new BS acknowledges the MT and adds it to a database it has of MT's for which it is responsible and begins to route communications to the MT accordingly. The new BS also informs the old BS that the MT is now located within its coverage area and may be reached through the new BS. The old BS then adjusts its database to forward any communications it receives for the second MT to the new BS. The new BS acknowledges to the second MT that the handoff is complete.
With this method, the elongation of the data path increases with increased terminal mobility. This elongation of the data path is seen with reference to FIG. 2 which shows a more complicated WLAN and LAN network 50'. This network includes six switches 54 (switches A-F). Three base stations 60g, 60h, and 60i are shown connected to switch C. One base station 60k is shown connected to switch F. The communication path 66 between MT.sub.g, initially located in coverage area 62g and MT.sub.k located in coverage area 62k is from BS 60g.fwdarw.switch C.fwdarw.switch A.fwdarw.switch F.fwdarw.BS 60k. As MT 64g moves through different coverage areas, the path 66 becomes increasingly elongated. As seen in FIG. 2, MT 64g moves from coverage area 62g through 62h and 62i and ends in 62j. BS 60.sub.j, which covers coverage area 62j, is connected to switch D. The path 66 becomes increasingly large (e.g., elongated) with each move in FIG. 2. (Note that the path 66 enters and exits switch C four times). Each time the communication is received by a BS, the communication is translated from the ATM cell format to the wireless transmission frame format. When the BS determines that the communication is not to be transmitted into the coverage area, but rather forwarded to another BS, the BS translates the communication back to the cell format and forwards it. The increased communication path 66 of this elongation method violates the third rule above, because it leads to delay, bandwidth waste, and other disadvantages contrary to the needs of high speed network transmission.
At the January 1995 ATM Forum, a regulation for "LAN Emulation Over ATM: Version 1.0 Specification" was developed. The Goodman et al. path elongation method complies with this regulation. The basic principle of LAN emulation (LE) is establishing a VCC between each base station pair so that all of the work stations on the ATM LAN and WLAN can communicate with each other through a VCC. The recipient MT's base station first assembles the ATM cells into wireless data frames and then transmit the frames to the MTs according to the address.
The LE Version 1.0 Specification is based on wired LAN properties, which allows a conventional connection free (e.g., wireless) media-sharing network architecture to connect easily with a connection oriented (e.g., wired) ATM network.
It is an object of the present invention to provide a seamless handoff method for a mobile terminal in a WLAN.
It is another object of the present invention to provide a handoff method that reduces the delay caused by the VCC path elongation.
It is yet another object of the present invention to provide a WLAN emulation system for use in an existing LE environment to increase the LE environment's compatibility and real-time efficiency.